【A-Level經濟精講】需求的收入彈性（Income Elasticity of Demand）

今天要給大家講解的是Elasticity of demand中的最后一個Elasticity：Income elasticity of demand，它所包含的知識點也比較細，所以建議同學們在仔細閱讀后分類記憶，同時也可以跟之前學習的PED以及XED進行比較和聯系，Elasticity這一塊的知識點都有些零碎，也要注意定時復習回顧哦。

Income elasticity of demand的定義：

It measures the responsiveness of quantity demanded when there is a change in income.

計算公式:

% of change in quantity demanded/% of change in income.

YED value分為兩種情況來討論，一種是Normal good，一種是Inferior good。Normal goodFor normal good, when the income of people is increasing, the quantity demanded for normal good will increase; When the income of people is decreasing, the quantity demanded for normal good will decrease.當收入增加，人們會購買更多的正常商品，

當收入減少，人們會購買更少的正常商品。

所以，Normal good的YED value一定是正數。

Inferior goodFor inferior good, when the income of people is increasing, the quantity demanded for inferior good will decrease; When the income of people is decreasing, the quantity demanded for inferior good will increase.
當收入增加，人們會購買更少的劣質品，當收入減少，人們會購買更多的劣質品。

所以，Inferior good的YED value一定是負數。

綜上所述，YED的Mathematical sign可以告訴我們Product是Normal good還是Inferior good。

同時，根據YED value的絕對值大小，又可以將其分成四類。（以下數字全部為assumption）1YED=3 (Income elastic)The product is normal good, e.g.luxury bags2YED=0.6 (Income inelastic)The product is normal good, e.g.necessities3?YED=-2.1 (Income elastic)The product is inferior good, e.g.frozen dinner4YED=-0.2 (Income inelastic)The product is inferior good, e.g.canned fruits

The extent(Absolute value) of YED tells us whether or not the QD of a product is sensitive to income change.

YED數值的絕對值告訴了我們一個產品對于收入是否敏感。

YED的用途

It can helps to decide the production level and nature of a firm.

如何決定Production level需要借助Economic cycle，如下圖：

當處于Growth/boom的時候Should increase the production of positive YED products, which are normal goods, luxury, necessities and decrease the production of negative YED products, which are inferior goods.

增加正常商品的生產，減少劣質品的生產。Recession/slumpShould decrease the production of positive YED products, which are normal goods, luxury, necessities and increase the production of negative YED products, which are inferior goods.

增加劣質品的生產，減少正常商品的生產。